The present invention, in some embodiments thereof, relates to compositions comprising fibrous polypeptides and polysaccharides and uses of same.
The most extensively investigated biological polymers for use in material science are polysaccharides due to their abundance and extremely diverse mechanical properties.
The polysaccharide cellulose is the most common biopolymer on earth. Although it is mostly found in plant biomass, it is also produced by animals, fungi and bacteria. Cellulose is a crystalline assembly of cellobiose subunits which are made from glucose. Due to its crystalline structure, cellulose has high tensile strength and elasticity approaching that of synthetic carbon fibers, and it has a very favorable strength/weight ratio compared to, for example, steel. In plant cell walls, cellulose is found as a composite with other polysaccharides such as hemicellulose, pectin, lignin, enzymes and structural proteins. These molecules link the cellulose microfibrils improving the mechanism of load transfer when the cell is subjected to mechanical stress whilst enhancing physical protection against pathogen attack.
The unique properties of natural biocomposites have prompted many scientists to produce composites of cellulose and synthetic polymer matrixes. For example, Favier et al, [Polymer engineering and science 37(10): 1732-1739] produced cellulose-latex composites resulting in increased shear modulus by more than three orders of magnitude of the latex rubbery state. Such biocomposites have been produced for the automotive industries and for production of biodegradable plastics.
The use of cellulose binding domains (CBD) for cellulose fiber modification is a well established technology [Shoseyov et al, Microbiol Mol Biol Rev. 70(2):283-95]. Recently, CBD was used for production of novel cellulose-protein composite materials when recombinant CBD or CBD dimers, CBD-CBD fusion proteins (CCP), were bound to paper resulting in improved mechanical and water repelling properties [Levy et al., Cellulose 9: 91-98]. Furthermore, a recombinant CBD-starch binding domain (CSCP) demonstrated cross-bridging ability in different model systems composed of insoluble or soluble starch and cellulose [Levy et al., Cellulose 9: 91-98].
In addition to polysaccharide research, biopolymer research has focused in recent years on fibrous proteins due to their unique mechanical properties. These proteins are distinguished by their repetitive amino acid sequences that confer mechanical strength or flexibility. Among these proteins are mammalian collagen and elastin and the arthropod proteins, silkworm silk (Bombyx morii), spider dragline silk and resilin. The unique repetitive sequence of each protein confers its mechanical properties. For instance, spider silk is extremely strong while resilin and elastin are extremely elastic and resilient with a rubber-like nature.
Resilin is found in specialized cuticle regions in many insects, especially in areas where high resilience and low stiffness are required, or as an energy storage system. It is best known for its roles in insect flight and the remarkable jumping ability of fleas and spittlebugs. The protein was initially identified in 1960 by Weis-Fogh who isolated it from cuticles of locusts and dragonflies and described it as a rubber-like material.
Resilin displays unique mechanical properties that combine reversible deformation with very high resilience. It has been reported to be the most highly efficient elastic material known. The elastic efficiency of the material is purported to be 97%; only 3% of stored energy is lost as heat (U.S. Patent Application 20070099231). Resilin shares similar mechanical properties with elastin which is produced in connective tissues of vertebrates. In humans, elastin is usually found at sites where elasticity is required, such as the skin and cartilage (often in association with collagen). Elastin-collagen composites also serve as a major component in arterial walls where it allows the blood vessels to smooth the pulsatile flow of blood from the heart into a continuous and steady flow.
In spite of their functional analogy, the sequence homology between resilin and elastin is very low, apart from the high abundance of glycine in both proteins. Nevertheless, the elasticity of both proteins results from their architecture of randomly coiled, crosslinked polypeptide chains. Resilin is synthesized in the insect cytoplasm and subsequently secreted to the cuticle where peroxidase enzymes catalyze its polymerization via formation of di/tri tyrosine bridges, resulting in assembly of a natural protein-carbohydrate composite material with cuticular chitin. Two Drosophila melanogaster Resilin mRNA variants have been identified—CG15920-RA and CG15920-RB which differ in the truncation of their chitin binding domains (see FIG. 1A). The major components that were annotated are the 17-amino acid long elastic repeats and the 35 amino acid-long chitin binding domain of type R&R.
Recently, Elvin et al., 2005, [Nature. 437: 999-1002] successfully expressed and polymerized a synthetic, truncated resilin-like gene in E. coli. The synthetic gene consists of the 17 repeats of the native gene. The protein, once expressed, undergoes photochemical crosslinking which casts it into a rubber-like biomaterial. U.S. Patent Application 20070099231 discloses hybrid resilins comprising resilin and structural polypeptides.
Silk proteins are produced by a variety of insects and arachnids, the latter of which form the strongest silk polymers on earth. The spider spins as many as seven different kinds of silks, each one being optimized to its specific biological function in nature. Dragline silk, used as the safety line and as the frame thread of the spider's web, is an impressive material with a combination of tensile strength and elasticity. Its extraordinary properties are derived from its composition as a semicrystalline polymer, comprising crystalline regions embedded in a less organized “amorphous” matrix. The crystalline regions consist of antiparallel β-pleated sheets of polyalanine stretches that give strength to the thread, while the predominant secondary structure of the amorphous matrix is the glycine-rich helix which provides elasticity. Most dragline silks consist of at least two different proteins with molecular masses of up to several hundred kDa. On the basis of sequence similarities, dragline silk proteins have been grouped into spidroin1-like (MaSp1) and spidroin2-like (MaSp2) proteins.
As opposed to silkworm silk, isolation of silk from spiders is not industrially feasible. Spiders produce silk in small quantities, and their territorial behavior prevents large amounts thereof from being harvested in adjacent quarters. Therefore, production of silk protein through recombinant DNA techniques is preferred. For such purposes, widespread use is made of synthetic genes based on a monomer consensus of the native spidroin sequences. These synthetic genes have been successfully expressed in the methyltropic yeast host, Pichia pastoris, in E. coli and in the tobacco and potato plants [Fahnestock S R., and Bedzyk L A Appl Microbiol biotechnol 47:33-39 (1997); Fahnestock S R., and Bedzyk L A, Appl Microbiol biotechnol 47:23-32 (1997), Sceller J. et al. Nature biotechnology 19:573-577 (2001)]. Through such means, laboratory scale amounts of silk-like protein powders are readily available. The final hurdle on the way to the production of manmade silks lies in the development of an appropriate spinning technology capable of converting these powders into high performance fibers. The tendency of these proteins to aggregate in-vitro, bypassing the protein folding process, acts as a significant limitation toward successfully producing functional silk. The assembly of the proteins from a liquid crystalline form into a solid silk string is extremely complex, and duplication of the operational function of spider spinning glands remains a major challenge.
Several attempts have been reported on the preparation of cellulose-silk fibroin composites which were prepared by molecular blending and regeneration of solubilized cellulose and silkworm silk [Freddi G, et al., (1995), J Appl Polymer Sci 56: 1537-1545; Yang, G, et al., (2000) J Membr Sci 210: 177-153]. Recently, Noishiki et al [Noishiki Y, Nishiyama Y, Wada M, Kuga S, Magoshi J. (2002) J Appl Polymer Sci 86: 3425-3429] prepared composite cellulose-silk films from solid cellulose whiskers and regenerated silkworm silk, resulting in notably improved mechanical strength, with breaking strength and ultimate strain about five times those of the constituent materials alone.